In general, automatic transmissions using a planetary gear or a transmission mechanism with parallel counter shaft are used in the conventional automatic transmissions, in which the clutches installed at the individual gear trains having their own different gear ratio are selectively engaged in order to shift gears. This prior art is disclosed, for example, in Japanese Patent Laid-Open Number 10-89456 (1998).
The result of analyzing the above prior art is described below. The analyzing result shown below is not a straight forward description of the prior art but shown as the analyzing result.
In case of shifting up gears, as the next-stage clutch is started to be engaged and the torque transmitting power is gradually made increase in a partial connecting condition, the torque transfer in the torque phase arises in which the transmitting torque of the pre-stage clutch gradually decreases. When the pre-stage clutch is released in synchronization with the timing when all the torque is transferred to the next-stage clutch, the rotating speed transmission in the inertia phase arises in which the engine speed decreases down to the input rotating speed of next-stage gear.
In case of shifting down gears, even if the transmitting torque of the next-stage clutch is made decrease, it is not principally possible to perform the torque transfer from the high gear position with a lower energy potential to the lower gear position with a high energy potential. For this reason, the rotating speed transfer is controlled at first in which the engine speed is made increase by making the pre-stage sliding, and then the torque transfer is controlled in which the clutch is made engaged in synchronized with the rotating speed of the next-stage clutch.
Thus, in the transmission control in the prior art, the torque transfer in the torque phase is managed by the friction control of the clutch, and the inertia energy is released in the inertia phase. However, in this method, there is such a disadvantageous aspect that the clutch plate is damaged due to its friction and thus its life time becomes inevitably short. In addition, according to this method, as the torque transmitting power is controlled by adjusting the friction force and the friction force has a negative resistance characteristics with respect to the relative speed, it is extremely difficult to control stably the torque transmitting power in a designated value, which leads to shift shock due to the generation of judder and even in the worst case, the clutch plate may be worn away with a rippled surface.
Especially in a shift down operation for accelerating the automobile by pressing the accelerator, as the torque transfer can not be realized in the beginning in principle, the rotating speed is controlled in advance and then the clutch is connected for the low speed gear before the torque transfer. For this reason, the response from pressing the accelerator to reaching a designated torque is so slow that the drivability is not good.